Limit control for motors



S@Pf- 24, 1963 c. B. lSANBORN, JR 3,105,181

I LIMIT CONTROL FOR MOTORS Filed Dec. so, 1959 4 QF-F lah, REcTnFR 'Rm-men N4 `cotoLuNlT coNTRoLuNn' E 1 az 35h l l r L Fi .l W3 Ll 3 Lz Ls l Mm j ML: 5-L5 A:

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United States Patent Office 3, l d5, l 8 l Patented Sept.. 24, l 963 3,105,131 LIMHT CONTROL FOR MOTORS Calvin B. Sanborn, Sir., Bedford, Ohio, assignor to Square D Company, Detroit, Mich., a corporation of Michigan Filed Dec. 30, 1959, Ser. No. 863,020 7 Claims. (Si. 3HE-203) This invention relates to a control system for controlllng the mode of operation of a motor which system includes a limit control system responsive to operation of the motor beyond a predetermined limit of operation, and for controlling accelerating and decelerating movements of a load, particularly in la motor and load combination.

In many instances, it is necessary to provide limit switches which will automatically discontinue energization of a moto-r and thus stop movement of a load beyond a certain point. Generally, it is necessary to provide a control circuit which, after the limit switch has been opened, will permit movement of the load out of the limit switch, or continue operation `of the motor in a direction opposite to that in which it moved the load into the limit switch. An example of such an instance is lIt is also advisable to provide a circuit which will permit driving down torques in case an empty hook is hoisted into the limit switch, because, in such cases, there would be no load on the hook to tend to drive the motor in a downward direction.

Another problem is that of being able to drive a load downwardly by energizing the motor ttor the downward direction off rotation and controlling the speed of that downward movement even in those cases wherein the limit switch will not return to its normal position after it has been tripped and Iafter the load has been again moved out of Contact with the switch. e

`One of the objects of the present invention is to lovercome the aforementioned deliciencies and problems encountered in the prior circuits.

Another object ofthe present invention is to provide a system for controlling acceleration and deceleration of an [overhauling load in such a manner that the load is repeatedly caused to accelerate and then decelerate in a controlled sequence of movement depending on the speed of movement of the load while the load is moving only in yone direction.

Another object of the invention is to provide a limit means in the control system of an electric motor to ef fect de-energization of the motor when it reaches a certain limit or loperation and to permit energization of the motor for an opposite direction of rotation.

A further object of the invention is to provide a control system for controlling the mode of operation of a multi-phase mot-or by controlling the energization of each of Athe field windings of the motor by multi-phase power and in which rst means are operative to eifect energization of each of the field windings by its respectivephase of the source of power to cause the motor -to operate in a first mode of operation, and in which limit control means are responsive to operation of the motor beyond a certain limit of operation in that iirst mode of operation to effeet de-energization of atleast one of the iield windings and to form a closed loop consisting of a current conductor means and at least another one of the field windings, and in which second means are operative while the limit switch is tripped to effect energization of at least one of the field windings by a phase of the source of power to cause the motor to operate in a mode of operation different from the `first mode of operation.

Another object of the invention is to set a friction brake when the limit switch operates.

Other objects and a fuller understanding of the present invention may be had by referring to the appended claims defining the present new, novel and useful invention or discovery, to the lfollowing description of a specific means or method contemplated by the inventor for carrying out his invention, and to the accompanying drawings in which:

FIGURE 1 is a schematic diagram of a system incorporating the invention; and

FIGURE 2 is a modification of the invention showing the adaptability of the limit switch with forward and reversing contactors in place of forward and reversing reactors.

The following description of the attached drawings and the accompanying claims jointly set forth one or more modiiications incorporating the present invention and the advancement in the art of motor control circuits. As used throughout the present description and claims, the specific terms used to identify the parts or components have been arbitrarily chosen to indicate to others commercially available parts or components which may by readily obtained to carry out the specific mode of the invention as described herein, and they are to be interpreted in their broad sense wherein they include electrically or mechanically equivalent components which will provide the same functions as those mentonedhere- In the following description, it is understood that the present illustration is for the purpose of exemplication and is not intended in any way to limit the scope of the invention either to the specic application illustrated or to the functions obtained thereby.

The individual components in the circuit are identified in the drawings either by letters or a combination of letters and numbers. It is further noted that where convenient, the letters used as reference characters to identify a component are, to some extent, an abbreviation of the function performed by the respective component in the control system.

FIGURE l illustrates thepcontrol circuit for controlling the speed and direction of rcniation of a three-phase induction motor l0 having a stator -11 and a rotor 12. The stator 11 -is provided with terminals T1, T2 and T3, which receive power from a three-phase source of voltage land current represented by supply lines Ll, L2 and L3. The stator has as many stator windings as there are phases in the source of power. Interconnected between the stator 11 and the lower lines L1, L2 and L3, are three individual and magneticaily separated saturable reactor units. The reactor units :are designed in accordance with well known design .pnnciples ior saturable reactors which are commonly used in the industry. One of the reactor units has alternating current power windings I-IXl and HXZ normally of high impedance and a direct current satnrating or control winding HXCW arranged on a com mon magnetic core (not illustrated) so that magnetic ilux established by the control winding HXCW threads both of the alternating current power windings HX1 land HXZ. Another reactor unit has alternating current power windings LX1 and LXZ normally of high impedance and .a Kdi-rect .current saturating winding LXCW, while the third reactor unit has only an kalternating current power winding BX normal-ly of high impedance, and a direct current saturating winding BXCW. It is noted that the direct current saturating windings HXCW, LXCW and BXCW are physically removed from each other so that ux generated .by energization of larry one of them is tree from threading through any other one or more of them. The impedance of each alternating current power winding is reduced in proportion to ran increase in the energizration of its respective direct current saturating winding.

The Ialternating current power winding HXl has end taps and 16; the alternating current power Winding HXZ has end taps 17 and 18; alternating current power f LX1 has end taps 19 and 20 and anintermedifate. tap Z1; and, Vthe alternating' current power winding LX2 has end taps 22 iand 23 and an intermediate tap 24. End taps 15 and 19 or yalternating current power windings HX1 and LX1, respectively, are interconnected to the power line L1 'and the end taps 13 and 23 of the alternating current power windings HXZ rand LXZ, respectively, 'are interconnected to the power line L2. The end'tapr 16 of alternating current power winding HX is connected to the intermedi-ate tap 24 of the alternating current power winding LXZ and the end tap 17 of the lalternatin-g current power winding HXZ is [connected to the intermediate tap 211 of the'alternating current power winding LX1. The end tap 2d is connected to the terother and are operative in such manner th-at a com-bina- `tion signal of the master and -feed back signal effects parallel connection is connected in series with the' direct minalT and the end tap 22 is connected to the terminal l T2. The alternating current power winding BX has one end tap 25 connected to the terminal T3 and another end tap 26 connected to the power line L3.

As also described in -a co-pending application SerialV No. 862,842 entitled Motor Control and filed of even date herewith, now U.S. `Letters Patent 3,076,126, issued January 29, 1963, energizatio'n of the direct current saturating windings -IXCW, VLXCVV and BXCW is in accordance with the algebraic sum and combination of a command signal and a feed 'back signal. The command signal isV ironia master 30 which defines a source of the commandsignal, and the feed back signal is derived trom' a feed back signal source, such as a speed responsive Vmeans which may be in the form of a dircct'current tachometergenerator31 driven by the motor 16x The command signal from the master 30 and the feed back signal-from the tachorneter generator 31 in this instance are both direct current voltage signals and there- 'fore are easily. combined by connecting the output terrninals of the master 30 and the tachometer generator-'31 in series electrical circuit connection. series connection, represented Vby the wires 32 and 33, are rectiiier control. units 34 and 35 which operate as described in the aforementioned patent in accordance with the combined signal provided by the master 3d and the tachometer generator 31 to controly energization off the direct current saturatin g windings HXCW and LXCW. Thus the rectifier control -means 34 and 35 provide a lmeans responsive to the command signal from the master 3o and the' feedback signal from the tachometer 31.V

The direct current saturating windings BXCW, HXCW and LXCW are energized by current from a current source V4t`i-wihichY in this instance provides alternating'current. For example, unidirectional ow of current through the direct current saturating Winding HXCW Interposed in this ing through LXZ.

current satunatingA winding BXCW. The combination series `and parallel `arrangement isthen connected across vthe' current ysource 4t?.V Unidirectional flow of current direct current sat-urating winding HXCW or the directVV current saturating winding LXCW, that same cu-rrent flows through the direct current saturating winding BXCW so that the magnitudeof current liowing Vthrough the alternating current power winding BX is controlled simultaneously with the controlling of the magnitude ot current flowing through either alternating current power n windings HXl and HX2 or the magnitude of'current flown alternating current power' windings LX1 and One of Vthe applications of the present motor control circuit is in hoisting wherein the motor 10 is to hoist a load or to control the lowering of the load. When the motor is used in this application, movement control means which, as illustrated, is in the form of a spring set electromagnetically released brake 6)l is usually applied Y' to the driving structure, .or mechanically coupled to the rotor 12 of the motor 1l).v FIGURE 1 illustrates such a movement controlling brake as having an energizable release coil 61 connected, preferably, inseries'with a safety switch `62, and between the end tap 1S andan intermediate tap 63 :on a voltage divider or coil 64 interconnected between the terminals T1 and TZ. Brakes of this type are commonly used in the industry and have a spring -65 which normally sets the'brake to. prevent rotation of the rotor 12. Energization of the coil 61 overcomes the spring 65 to permit rotation of the rotor. With this circuit arrangement, ,the energizable release coil 61 is energized whenever the switch I62 is closed and power is appliedto the motor terminals T1 and T2. l

The magnitude of the current to these terminals, T1 and T2 or the phaservoltages impressed thereon, has no effect on the magnitude of .the voltage impressed across the'coil y61. The voltage across the coil 61 is approximately one-half the voltage acrossV phase L1-L2. Although the safetyk switch 62V has been illustrated as a push button for purposes of clarity, it is understood that it can be interlocked with the operating handle 30h of VVthe master `3d in vanymanner commonly used in the art.Y

` `When thesafety switch-62 is used, it mus-t be held down is provided by connecting it as the load across a full-wave s bridge rectiiier circuit including rectiiiers 41 and 42 and control means such as controlled rectifiers4-3 and 44. The rectiiiers are connected inthe bridge in such manner that any' instant of time, current iiows either through rectifier 43 or 44 and not through both of them simuly taneously. The magnitude of current allowed toni'low through the controlledrectifiers 43 and 44 is controlled 'by the rectifier controlV unit 34 by connecting thev control elements 43g and 44g `of controlled rectiers 43 and 44 f to the rectifier control unit 34. .Y

Rectifier control units 34 and 35 are similarY to' each Ymally open contacts and closedY to permit energization of coil 61 and release of the brake to .operate `.the hoist by moving'operating handle 30h in any of its hoist and lower positions wherein the master 36 provides the desired hoist or lower command signal.l l n FIGURE 1 further illustrates the featureof using a limi-t switch 7d', operable by-hoisting of a load into the limit switch, to, prevent continued hoisting of the load' and continued. operation lof the motor in the hoisting direction regardless of the command 'signal being given by the masterd. Limi-t switch 76 is of any well-knownV design commonlyused in the industryrand includes noropen contacts LS1 and LSZ are yclosedfand the normally Y LS1 and L52 and normally kclosed i, contalctsLSS and L54. The switch 76 is operative in Y such manner that when the loadV engages it, the normally i closed contacts LS3 and LS4 are opened. The normally open contact LS1 is interconnected directly between the supply line L2 and the terminal T2 so that it shunts the alternating current power winding LX2. The normally open contact L52 is interconnected in series with a current conductor means 71 between the terminals T3 and T1. The normally `closed Contact LSG- is interconnected in series with the alternating current power winding LX2 and in shunt with contact LS1. The normally closed contact L54 is interconnected in series with the alternating current power winding BX and the terminal T3. The current conductor means '71 may be physically constructed in the form of ia resistor, capacitor, reactor means, either singly or in combination, all of which are considered electrically conductive devices which will .conduct current and at the same time do not necessarily form 'a short circuit between the terminals Tl and T3.

Operation With the initial movement of the master and particularly handle 30h in the hoist direction, the mas-ter switch produces =a direct current command signal which is fed into the rectifier control unit 34 and the rectiiier control unit 3S. This direct -current signal turns on the hoist rectifier control unit I34 and leaves ofi the rectifier control unit 35. The turning on of the hoist rectifier control unit 34 causes the controlled rectiliers 43 and 44 in the rectier bridge circuit to supply direct current to the direct current saturating winding HXCW. At this time, no current flows through direct current saturating winding LXCW ror'through power windings LX1 and LX2 because rectifier control unit 35 has not been turned on. The tachometer generator 31 produces no feed back signal because the hoist has not started to move.

Current is now conducted from the yalternating current supply 40 through the bridge rectifier energizing the direct current saturating winding HXCW and the bridge rectifier energizing the direct current saturating winding BXCW. The direct current iiowing through direct cnrrent saturating windings HXCW and BXCW partially saturates the respective saturable reactor units, reducing the impedance of the respective alternating current windings HXd, HX2 land BX. With the saturation lor lowering of the impedance of alternating current windings H1, HXZ and BX, the potential at the terminal Tl approaches the potential at the supply line L2, the potential at the terminal T2. approaches the potential at supply line Ll, the potential at the terminal T3 approaches the potential at the supply line L3. When the impedances of alternating 'current power windings HXl, HXZ and BX yapproach zeno, the terminals T1, T2 and T3 are at substantially the same potential as the line terminals Li., LZ, `and L3, respectively. Adjustment of the impedance of the alternating current windings adjusts the voltage at Le terminals T1, T2 and T3 and thereby the speed of the motor in the hoist direction.

As shown in FIGURES l and 2, the end tap 16 of the alternating current power winding ylrlXl is fastened to the intermediate tap 2li of the alternating current power winding LX2 and the end tap 17 of the alternating current power winding HXZ is fastened to the intermediate tap 21 yot the alternating current power winding LX1. Because or" this, the alternating current power windings LX1 and LX2 operate 4as an autotranstormer to produce a voltage in their overhanging sections, i.e. between the intermediate tap 21 land the end tap 2.0 and the end tap Z2 yand the 4"mtern'iediate tap 24 respectively. The voltage in the'cverhanging winding between the end tap 2t) and the intermediate tap Z1 of alternating current power winding LX1 is in the same direction as the voltage in alternating current power winding HXl at any instant of time. The voltage in the overhanging winding between the end tap Z2 and the intermediate tap 24 of the l alternating current power winding LX2 is in the same direction as the voltages in the alternating current power winding HXZ at any instant of time. The voltage added by the overhangs replaces voltage drop across the alternating current power windings HXl or HXZ due to impedance inherent in the windings which cannot be overcome by the winding HXCW. In this way, the terminal voltages may be made substantially equal to the line voltages during hoisting and when the master is commanding full speed hoist. The terminals T1, T2 and T3 may be considered to have balanced voltages applied thereto, even though the voltage loss in the alternating current power winding BX has not been compensated for.

When the load starts to move in the hoist direction, the tachometer generator 31, which is mechanically coupled to the motor shaft, starts to produce .a feed back voltage signal which is proportional tothe speed of the motor it?. This feed back signal from the tachometer generator 3l opposes the direct current output voltage signal from the master switch 30, and the combination controls the hoist and lower rectifier control units 34 and 3S. Since the motor is moving in the hoist direction, these command and feed back signals will keep the lower rectifier control unit 35 effectively turned off. At the same time, the hoist rectifier control unit 34 output is reduced to a point at which the current fiowing lthrough the direct current saturating winding HXCW causes the voltage applied to the moto-r terminals to establish a motor torque sufficient to hoist the` load at the speed determined by the master switch 3%. The speed of the motor stabilizes when the signals from the master switch 3i) and the tachometer generator 3i effectively combine to indicate tothe rectifier control units that the motor is operating at the desired speed and in the desired direction.

The hoist is now lifting the load at the speed selected by the setting of the master 30. Adjusting the master 30, but keeping it in the hoist range, will cause the motor to run at any one of an infinite number of stable hoisting speeds. The motor operates at the various speeds because the various phase voltages applied to the motor may be varied from zero to the line phase voltages L1, L2 and L3. With the line phase voltages all applied to the terminals, the motor will run at a maximum obtainable speed in accordance with the weight of the load. Light loads can be lifted at a faster maximum speed than heavy loads.

lf the load is hoisted into the limit switch 70, the limit switch operates to close normally open contacts LS1 and LSZ and open normally closed contacts LSB and L84. Closing the normally `open contacts LS1 connects the terminal T2 directly to supply line L2, and closing the normally open contacts L82 connects the current conductor means 71 across the terminals Tl and T3. Simultaneously, the normally closed contacts LS3 open to prevent any possible current flow .through the alternating current power winding HXl and at least a portion of the alternating current power winding LX2 between supply line L1 and the terminal T2 and also to prevent current ow through the alternating current power winding HXl and a portion of alternating current power winding LX2 and the normally open contacts LS1 between `supply lines L1 and L2. Normally closed contacts L84 open to effectively disconnect terminal T3 from the supply line L3. At

this time, and even though master 30' may still be providing a hoist command signal, the motor is effectively deenergized and the brake 60 will set. The motor is effectively de-energized because the terminal T3 is disconnected from the supply line L3 and the terminal T1 is electively disconnected from the supply line L1 due to the impedance of the alternating current power winding LX1. The brake `60 will set for lack of voltage drop across voltage divider 64 and between the intermediate tap 63 of the voltage divider 64 and the supply line L2.

lf the master 3@ is now returned to the off position so as not to provide a command signal, the brake will remain set and the motor will be de-energized the same as 7 the normal operation when the load has not tripped the limit switch.

If the master 30 is operated to provide a lower command signal which directs operation of `the motor in a l lowering direction, rectifier control units 3ft and 35 will cause energization of the direct current saturating winding LXCW, but not of the direct current saturating winding HXCW, therebyreducing the impedance of the alternating current power winding LXI and causing the alternating current power winding HXl to lbe a high imf pedance. This affects energization of at least one of the windings of the statory 11 `because current can now flow in a standby circuit through the now closed limit switch contacts LSli and the now saturated reactor LX1, between supply lines L1 and L2 to terminal T1 and terminal T2. In addition, a voltage drop again appears across voltage .divider ovwith a result that coil 61 is energized and the vbrake releases, providing the switch 62 is closed. With the brake'released and the voltage applied to motor terminals T1 and T2, or at leastA one phase of the stator windings, and with terminals T1 and T3 of the motor shunted bycurrent conductor means 71, the motor will rotate in the downward direction and the load will move downwardly and away from the limit switch '7th Itis noted that the current flowing in the current conductor means `'71 and between terminals T1 and T 3 is out yof phase with thecurrent flowing through terminal T l vand T2 ,fromV the power linesLl and L2. This causes a downward torquel on the rotor 12. After the load has moved out of the limit switch dit, the contacts'LSl and LS?, again are-open and LS3 andrLSll are again closed so that the circuit is returned for normal operation. However, if the limit switch 'fails to reset itself `and if the load is sufciently large to become koverhauling in which case it overcomes all the frictions in the motor and driving units and actually drives the motor in a'lowering direction, the rectifier control units respond-to the combination of the command and feed back signals to control that downward movement. In effect, the lower command signal from the master 30 causes the brake to be released, assumingV switch 62 is closed, and the motor llt to run or -be driven in the lowering direction by the load until the 'tachometer generator 31 has accelerated to a speed when its feed' MH which is operative when energized to close the sets of contacts MH1 and MHZ. The'reactor windings LX1 and LX'Z have been replaced by normally open sets of contacts MLlt and ML2 and the winding LXCW has been replaced byr a contacter coil ML which is operative when energized to close the sets of contacts MLll and ML2.

'The reactor .winding BX has been replaced by normally open set of contacts ML and the winding BXCW has been replaced `by a contacter coil M which is operative when energized to close the set of contacts M1. The operation of the limit switches, the master, the tachometer generator and the rectier contr-ol unit is the `same as in FIGURE. 1. Therefore, the operation of the circuit of FiGURE 2 is similar to that of FiGURE l insofar as the limit switches are concerned. However, it is noted that energization of the motor for a particular direction ofrotation is either at full line voltage or at zero voltage whereas in the circuit of FIGURE l, the motor may be variably energized depending on the magnitude of the command andk feed backV signals. v

The control vcircuitry described provides regulator means. It is apparent from the foregoing that when the load Vis an overhauling load,*the motor and the control circuitry pro-vide the regulator means. On thefother hand,

when the motor itself Vis the prime mover, it andthe con- Y to without departing from thespirit and the scope of the invention ashereinafter claimed.

What is claimed is: Y

1. A controlk system for controlling the mode of operation lof a multi-phase motor by controlling the energization, from a multi-phase `source of power, of each of the windings ofthe motor, and Acomprising first means operaf tive to effect energization ofthe motor windings by the back signal overcomes the lower command'signal.V At the Y instant the feed back Vsignal overcomes the. command lsignal and because the command and feed back signals are of opposing polarity, rectier control unit 35 responds to the two signals to completely de-energize' LXCW and simultaneously rectifier control unit 34 energizes HXCW.

De-energizing ,the `direct current saturating kwinding VLXCW removes energizing power from the motor,

and energizing the direct current saturating winding HXCW has no effect on the energization of the motor.

loss of voltage in the voltage divider 64 and a resultant setting of the brake 6ft' therebydecelerating and stopping movement of the load. y Y

As soon as the load has stopped moving and the corn-v mand signal has again overcome the feed back signal, the

' direct current saturating winding LXCW is again energized and the complete cycle starts over. The cycle ofk alternate acceleration and deceleration4 continues to repeat itself until the loa-d has come to a safe resting position on some supporting structure. Thus the motor and its control regulate the downward movement of the load, even when the limit switch fails to reset.

The limit switch arrangement as used in' FIGURE l ymay also be used in connection with an electromagnetic device, such as the reversing contactors in FIG. 2, .as well as with the reversing reactors of FIGURE 1. FEGURE 2 illustrates a circuit wherein the limit switch is applied in connectionrwith reversing contactors. In FlGURE 7.,Y ythe reactor windings HXi and HXZ have been replaced by normally open sets of contacts MHl and and the Winding HXCW has been replaced by a contacter coil During this transition period, it is noted 'thatthere isa source of power to ycause the motor to operate in a rst Inode of operation, current conductor means, limit control means responsive to operation of said motor beyond r`a certain limit of operation in ysaid first mode of operan tion to discontinue the energization of said motor windings v `by said source of power and to form a closed loop in, cluding said current'conductor means and at least one of' said motor windings, a standby circuit, said limit control Vmeans also being operable concurrently with discontinuance of `energization ofthe motor windings to connect a motor winding extraneous of the closed loop for subsequentr energization by said standby circuit, and second means operative while said limit control means is tripped to close said standby circuit,`thereby to reverse energiza- Ytion of the motor windings to cause said motor to operate in a `different mode of operation. n

2. The structure as defined in claim lY whereinsaid current conductor means includes a current resistor.

3.- TheV structure as delned'in claim 1 wherein said currentr conductor means includes a capacitor. v

vil. The structure according to claim 1 wherein the motor isa three-phase motor. n n 5. A vcontrol system for controlling the mode of operation of a multiphase motor by controllingl the energization ofthe terminals vof the motor 'by line'terminals, re-

spectivel'y,` of a multiphase source of power, and compris` ing current conductor means in an open loop circuit With two of said motor terminals, an energizable device associated with the motor and operative when Vfree of energization to affect the mode offoperation of the motor and operative when energized to be free from affecting said mode Vof. operation, first means operative in one condition to effect energization of the motor terminals by the source of power and to energize said device and motor to cause the motor to operate in a iirst mode of operation and operative in another condition to eect deenergization of said motor terminals and device to cause said motor to operate in a second mode of operation, limit control means responsive to the operation of said motor beyond a certain limit of operation in said rst mode of operation and while said iirst means are in said one condition to discontinue the energization of said motor terminals including said two motor terminals by their respective line terminals and close said loop, and to etiect deenergization of said energizable device to cause the motor to operate in said second mode of operation, a standby circuit, said limit control means also being operable concurrently to electrically connect one of the motor terminals other than said two terminals for subsequent energization by said standby circuit, and second means operative while said limit control means is tripped and said first means is in said another condition to close said standby circuit, thereby to effect reverse energization ofthe motor terminals to cause said motor to operate ina third mode of operation different from said iirst and second modes of operation.

6. A control for controlling rotation of the rotor of a three-phase motor by controlling the energization of the stator terminals of the motor from line terminals, respectively, of a three-phase source of power and comprising a resistor, a brake associated with the motor and having a spring operative to arrest rotation of the rotor and having a brake releasing coil energizable to overcome the spring to permit rotation of the rotor, iirst means operative in one condition to effect energization of each of the stator terminals by the line terminals and to energize the brake releasing coil to permit the rotor to rotate in a first direction and operative in another condition to effect deenergization of each of said stator terminals, and concurrently to deenergize said brake releasing coil, limit switch means operative in a tripped condition, only while said rst means is in said one condition and by rotation of the rotor beyond a certain limit of rotation in said rst direction, to discontinue energization of said stator terminals to connect said resistor to shunt with two of said stator terminals, and to eiect deenergization of said brake releasing coil to cause the spring to arrest rotation of the rotor, a standby circuit, said limit switch means a-lso being operable concurrently to electrically connect one `of the stator terminals other than said two stator terminals for subsequent energization by said standby circuit, and second means operative while said limit switch means is in said tripped condition and when said first means is in said another condition to close the standby circuit thereby to effect reverse energization `of all of said stator terminals to cause said rotor to rotate in a direction opposite said iirst direction.

7. In a hoist and control system in which the hoist is subject to overhauling loads, a load support, a motor drivingly connected to the support for lifting and lowering the support, hoist voltage adjuster means connected to the motor for controlling the magnitude of voltage applied to the motor for operating the motor in the hoisting direction when the hoist voltage adjuster means are connected to a source of power, lowering voltage adjuster means connecte-d to the motor for controlling the magnitude of voltage applied to the motor for operation of the motor in the lowering direction when the lowering voltage adjuster means are connected to said source of power, regulator means responsive to the speed of movement `of the support for controlling both of said voltage adjuster means, a spring applied electromagnetically released brake energizable by said source of power concurrently with energization, by each voltage adjustermeans, of the motor, limit control means adapted to be tripped by hoisting vof the load support by the vmotor beyond a pre-determined limit and normally connecting the motor and brake to both voltage adjuster means, and operative when tripped t0 render the motor and brake unenergizable through the voltage adjuster means, a bypass circuit operative when closed for rendering the motor and brake energizable through the lowering voltage adjuster means while rendering the hoisting voltage adjuster means inoperative to energize the motor and brake, and means operated by the limit control means, when the limit control means is tripped, for closing the bypass circuit, whereby if the limit control means is not reset, the brake is app-lied and released in response to the speed responsive means during lowering of `an overhauling load.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. A CONTROL SYSTEM FOR CONTROLLING THE MODE OF OPERATION OF A MULTI-PHASE MOTOR BY CONTROLLING THE ENERGIZATION, FROM A MULTI-PHASE SOURCE OF POWER, OF EACH OF THE WINDINGS OF THE MOTOR, AND COMPRISING FIRST MEANS OPERATIVE TO EFFECT ENERGIZATION OF THE MOTOR WINDINGS BY THE SOURCE OF POWER TO CAUSE THE MOTOR TO OPERATE IN A FIRST MODE OF OPERATION, CURRENT CONDUCTOR MEANS, LIMIT CONTROL MEANS RESPONSIVE TO OPERATION OF SAID MOTOR BEYOND A CERTAIN LIMIT OF OPERATION IN SAID FIRST MODE OF OPERATION TO DISCONTINUE THE ENERGIZATION OF SAID MOTOR WINDINGS BY SAID SOURCE OF POWER AND TO FORM A CLOSED LOOP INCLUDING SAID CURRENT CONDUCTOR MEANS AND AT LEAST ONE OF SAID MOTOR WINDINGS, A STANDBY CIRCUIT, SAID LIMIT CONTROL MEANS ALSO BEING OPERABLE CONCURRENTLY WITH DISCONTINUANCE OF ENERGIZATION OF THE MOTOR WINDINGS TO CONNECT A MOTOR WINDING EXTRANEOUS OF THE CLOSED LOOP FOR SUBSEQUENT ENERGIZATION BY SAID STANDBY CIRCUIT, AND SECOND MEANS OPERATIVE WHILE SAID LIMIT CONTROL MEANS IS TRIPPED TO CLOSE SAID STANDBY CIRCUIT, THEREBY TO REVERSE ENERGIZATION OF THE MOTOR WINDINGS TO CAUSE SAID MOTOR TO OPERATE IN A DIFFERENT MODE OF OPERATION. 